More specifically, the invention relates to those of these techniques whereby the sheets of glass are conveyed one by one through a heating furnace so that their temperature can be raised to a temperature near to the softening temperature, the sheets of glass being conveyed on a bed of rollers. The sheets of glass are then led, as soon as they leave the furnace, to a bending station in which a lower form lifts up a sheet of glass to press it against an upper form to obtain the desired shape. The sheet of glass is then set back down on the conveyor to be led to the cooling station such as a toughening station.
These bending techniques are described in particular in document U.S. Pat. No. 4,872,898. That document particularly describes a technique whereby a sheet of glass is transported on a roller conveyor through a tunnel furnace in which its temperature is raised to its softening temperature, and is then led to a bending station. In the bending station, the sheet of glass is lifted up from the conveyor by a frame which has the shape that the sheet of glass is to be given. The frame is discontinuous so that it can pass through the bed of rollers on which the sheet of glass initially rests. The frame then lifts the sheet of glass up to press it against a solid upper form, the shape of which complements that of the frame and therefore corresponds to the desired shape for the sheet of glass. After pressing, the lower frame drops back down to a level below the bed of rollers and thus sets the sheet of glass back down on said bed of rollers. The rollers then resume their movement to lead the sheet of glass to the toughening station.
Other documents describe variations on this type of technique. In particular as regards the lower frame and, more especially, its configuration to allow it to pass through the bed of rollers. Other documents describe even more variations on the designs of the rollers within the bending station, always with a view to facilitating the passage of the lower frame.
Aside from the special feature of the lower frame which passes through the bed of rollers which acts as a conveyor, this type of technique is characterized in that the bending operation is performed outside the furnace or at the very least outside of an enclosure which is maintained at temperature. This type of technique has therefore to be considered as a cold technique, this nomenclature defining the fact that the bending station is sited outside an enclosure that is maintained at temperature. What this means is that it is easier to control the positioning of the bending tools than it is in the case of hot techniques and that, on the other hand, the bending method is a race against time because as soon as it leaves the furnace, the sheet of glass will start to cool. Modifications to the bending operation or to its conditions are therefore tricky and limited.
Furthermore, recent developments, particularly in the automobile industry, are tending toward an ever increasing demand for glazings of a complex shape, particularly with very pronounced curvature. At the same time, the optical quality demanded is becoming ever higher. What is more, the thickness of the glazing is also reducing.
As regards the shaping of the lateral glazing for the motor industry on a tool using this technology, the direction of travel of the sheets of glass as they pass through the furnace on the roller conveyor is dictated by later usage considerations. Specifically, it is not desirable for marks to appear in a direction that is essentially vertical when the sheet of glass is in the use condition, which marks may be left by the rollers as the temperature increases; if such marks exist, they actually become highly visible from certain angles to an observer who is, for example, facing the motor vehicle. Thus, the direction in which said sheets of glass travel is dictated by the fact of ensuring that these marks are seen in an essentially horizontal direction and are therefore practically invisible to an observer.
Present-day demands by manufacturers are also imposing significant curvatures in the vertical direction after the sheet of glass has been mounted on the motor vehicle. This significant curvature will therefore correspond to a direction of bending corresponding to that of the conveyor in the furnace. It is thus tricky to be able to transport the sheets of glass after bending on a roller conveyor, these rollers being perpendicular to the direction of bending, without the risk of breaking or deforming the sheets of glass, these then remaining supported only by their central part. It is also desirable, particularly for good process repeatability, to maintain the shape obtained after pressing and not allow it to change between the pressing station and the cooling station.
Solutions for preventing deformation of the sheet of glass after it has passed through the pressing station have already been proposed. Document EP 0 523 017 in particular describes the use of a shuttle or moving frame which has the final shape of the sheet of glass for leading the latter from the bending station to the cooling station. Document U.S. Pat. No. 4,433,993 also provides a shuttle which takes a direction traversal to the direction of the conveyor passing through the furnace. These techniques allow the sheet of glass to be taken in hand immediately after pressing, so as to maintain the shape obtained and rapidly lead the sheet of glass to the cooling zone. These techniques do, however, have drawbacks; first of all, the travel of the shuttle entails indexing for each transfer operation, so that the sheet of glass is perfectly received. Furthermore, as the sheet of glass is supported at its periphery by a frame, cooling is not uniform; this disadvantage is particularly troublesome when the sheets of glass are annealed or semi-toughened for producing laminated glazing, or alternatively toughened. What happens in these various scenarios is that problems arise at the edges and lead to weakening of the glazing.